Photochemical smog, which is commonly characterized by ozone concentrations in the order of 0.1 ppm or greater in air, is an air quality problem in many urban areas, particularly those with high levels of sunlight.
Photochemical smog formation passes through three sequential phases: (1) oxidation of NO to NO.sub.2, (2) production of O.sub.3, and (3) a final phase when O.sub.3 is maintained at, or near, its maximum amount. Chemical processes occurring during each phase are intimately related and interactions between various competing and consecutive chemical reactions make analysis of smog formation difficult. Also because the atmosphere is in a state of dynamic flux since, as well as changing dispersion variables, there are changing emissions and changing meteorological conditions, e.g. sunlight, rain and temperature. In the atmosphere smog formation does not always reach completion because reactants are often dispersed before the final phase.
As a consequence of the above difficulties there is presently, despite considerable prior research efforts, a need for systems and methods which can provide reliable measures of smog formation in the atmosphere.
The commonly employed measure of smog concentration, ozone concentration, gives only a partial indication of the amount of smog formation. This problem arises because many chemical species, in addition to ozone, are products of the smog forming reactions and also because ozone is not a stable compound and is readily consumed, especially by reaction with nitric oxide. Thus at any given time the observed concentration of ozone in air is dependent upon the amount of prior emissions of nitric oxide into the air.
Considerable efforts by the inventor have thus been directed towards developing a robust method for determining the amount of photochemical smog formation in air and a laboratory size smog chamber which provides reproducible and accurate estimates of the photochemical reactivity potential of air being tested therein. However, it has been found that prior art smog chambers are prone to give irreproducible and inaccurate results which are thought to be due to different contributions from many variables, e.g. nature of the chamber walls and surface reactions therewith, shaded zones in the chamber, mixing rates, outgassing, chamber pretreatment, chamber deposits, impurities in reactants, non-uniform temperatures, etc, and smog reaction rates that are dependent on the extent of reaction.
There is also a need for a method for predicting smog formation from Reactive Organic Compounds (ROC)/air mixtures. Such a method could be used for screening solvents and fuels and assessing the photoreactivities of hydrocarbon wastes.
Photochemical smog formation from reactive organic compounds (ROC)/nitric oxide/air mixtures occurs as follows: ##STR1##
Present methods for measuring the essential reactant, ROC, which is essential for smog formation in the atmosphere are inadequate since they are either not sufficiently sensitive, are very cumbersome and labour-intensive or do not take account of the widely differing smog forming reactivities of the individual organic species which taken together comprise ROC. Frequently, the atmospheric concentrations of the individual ROC species, while sufficient to produce significant quantities of photochemical smog, are too small to be detected by the currently available sensors. Air can be analysed for ROC by high resolution gas chromatography using flame ionisation or photoionization detectors but these techniques require cumbersome sample preconcentration procedures, are labour-intensive and give data on only a subset of the photochemically active species present.
Furthermore, knowledge of the concentrations of the components of the ROC mixture does not allow the photochemical reactivity of the air to be quantitatively estimated because the role of many of the individual ROC species, and their reaction products, in the chemistry of smog formation, is uncertain. An approach sometimes adopted for ROC analysis is to measure the total non-methanic hydrocarbon concentration of the air as a single peak, backflushed from a chromatographic column after methane has been eluted, or alternatively as the difference in signal from air and air scrubbed of ROC species but without methane removal.
For some purposes this arrangement provides adequate sensitivity but the method is subject to errors in the measured concentrations because no account can be taken of the differing sensitivities of the detector to the various individual ROC species.
In other words, these techniques typically do not provide a measure of the reactivity of the total ROC in air since they do not provide their ROC compositions and this is important since in the atmosphere 250 or more ROC species have been identified of which there can be 60 major species or more and the rate of smog formation can be greatly affected by the nature and the relative proportions of the ROC species which are present.
Photochemical smog formation is a complex process wherein a multitude of reactant species are simultaneously consumed to give a wide variety of chemical product species. It is possible to measure the concentrations of many of these reactants and products and in the past such measurements have been utilized in various ways as indicators of the extent of photochemical smog production. For example, some measures that have been used to evaluate extent of reaction are: ozone concentration, peroxyacetyl nitrate concentration; nitrogen dioxide concentration; time to reach a maximum ozone concentration; time taken for the concentrations of NO and NO.sub.2 to be equal, ozone concentration attained after illumination for a fixed period and intensity; time for NO concentration to reach one hair of its initial value. Such data, however, gives only a limited indication of the progress of reaction and are complex and difficult to interpret in terms of the overall rate and extent of the smog-forming reactions. Additionally the rates at which these individual reactants and products are consumed and produced vary as smog formation progresses.